The present invention relates to a method of determining a voicing probability indicating a percentage of unvoiced and voiced energy in a speech signal. More particularly, the present invention relates to a method of determining a voicing probability for a number of bands of a speech spectrum of a speech signal for use in speech coding to improve speech quality over a variety of input conditions.
Development of low bit rate (4.8 kb/s and below) speech coding methods with very high speech quality is currently a popular research subject. In order to achieve high quality speech compression, a robust voicing classification of speech signals is required.
An accurate representation of voiced or mixed type of speech signals is essential for synthesizing very high quality speech at low bit rates (4.8 kb/s and below). For bit rates of 4.8 kb/s and below, conventional Code Excited Linear Prediction (CELP) does not provide the appropriate degree of periodicity. A small code-book size and coarse quantization of gain factors at these rates result in large spectral fluctuations between the pitch harmonics. Alternative speech coding algorithms to CELP are the Harmonic type techniques. However, these techniques require robust pitch and voicing algorithms to produce a high quality speech.
Previously, the voicing information has been presented in a number of ways. In one approach, an entire frame of speech can be classified as either voiced or unvoiced. Although this type of voicing determination is very efficient, it results in a synthetic, unnatural speech quality.
Another voicing determination approach is based on the Multi-Band technique. In this technique, the speech spectrum is divided into various number of bands and a binary voicing decision (Voiced or Unvoiced) is made for each band. Although this type of voicing determination requires many bits to represent the voicing information, there can be voicing errors during classification, since the voicing determination method is an imperfect model which introduces some xe2x80x9cbuzzinessxe2x80x9d and artifacts in the synthesized speech. These errors are very noticeable, especially at low frequency bands.
A still further voicing determination method is based on a voicing cut-off frequency. In this case, the frequency components below the cut-off frequency are considered as voiced and above the cut-off frequency are considered as unvoiced. Although, this technique is more efficient than the conventional multi-band voicing concept, it is not able to produce voiced speech for high frequency components.
Accordingly, it is an object of the present invention to provide a voicing method that allows each frequency band to be composed of both voiced and unvoiced energy to improve output speech quality.
According to the present invention, a voicing probability determination method is provided for estimating a percentage of unvoiced and voiced energy for each harmonic within each of a plurality of bands of a speech signal spectrum.
Initially, a synthetic speech spectrum is generated based on the assumption that speech is purely voiced. The original speech spectrum and synthetic speech spectrum are then divided into plurality of bands. The synthetic and original speech spectra are then compared harmonic by harmonic, and each harmonic of the bands of the original speech spectrum is assigned a voicing decision as either completely voiced or unvoiced by comparing the error with an adaptive threshold. If the error for each harmonic is less than the adaptive threshold, the corresponding harmonic is declared as voiced; otherwise the harmonic is declared as unvoiced. The voicing probability for each band is then computed as the ratio between the number of voiced harmonics and the total number of harmonics within the corresponding decision band.
In another embodiment of the present invention, the signal to noise ratio for each of the bands is determined based on the original and synthetic speech spectra and the voicing probability for each band is determined based on the signal to noise ratio for the particular band.